# Passively stable distribution of polarisation entanglement over 192 km of deployed optical fibre

## TL;DR

This paper demonstrates passively stable distribution of polarization-entangled photon pairs over 192 km of deployed optical fiber, achieving high fidelity without active stabilization, advancing long-distance quantum communication.

## Contribution

It shows long-distance entanglement distribution over deployed fibers with passive stability and high fidelity, eliminating the need for active stabilization or dispersion compensation.

## Key findings

- Entanglement fidelity of 85% over 192 km
- Stable quantum state over several hours
- No active stabilization or dispersion compensation used

## Abstract

Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85% $\pm$ 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre.